1. Field of the Invention
The present method and apparatus generally relate to a system for the simultaneous transmission and reception of signals over a single transmission medium utilizing a common path for both transmission and reception, and more particularly, to a bi-directional signal coupler for use with the single transmission medium.
2. Discussion of the Related Art
In the field of radio frequency (RF) interconnections, there exists a strong need to have devices, referred to herein as bi-directional couplers, for coupling both a transmitter and a receiver to a single communication medium. In a typical bi-directional transmission/reception communication system 10 as shown in FIG. 1, a transmitter 12 and receiver 14 operate simultaneously and at the same frequency for sending and receiving signals, respectively, over the communication medium 16. In the latter instance, signals are only differentiated by the direction in which the respective signals are proceeding. A bi-directional coupler 18 couples transmitter 12 and receiver 14 to the communication medium 16. A second bi-directional coupler 20 couples a second receiver 22 and second transmitter 24 to communication medium 16. Signals transmitted by transmitter 12 are send across communication medium 16 and received by receiver 22. Similarly, signals transmitted by transmitter 24 are transmitted across communication medium 16 and received by receiver 14.
With reference still to FIG. 1, two bi-directional couplers 18 and 20 are shown connected to each other via the communication medium 16. The communication medium 16 can be either a wired communication medium or a wireless communication medium. An exemplary wire medium includes a current-conducting medium. An exemplary wireless medium includes the use of RF (radio frequency) waves transmitted and received over a non-conductive path such as air. Other communication mediums are also contemplated, including optical fiber and such mediums as may additionally require the use of encoders, decoders, modulators, and/or demodulators, or other devise(s) necessary to facilitate the use of or interface the bi-directional coupler with the actual medium.
Major disadvantages and/or problems which occur in bi-directional coupling devices (18, 20), such as illustrated in FIG. 1, include insertion loss, rejection ratio, and parts precision. Insertion loss can be characterized, for example, in terms of the amount of transmitted energy from transmitter 12 that gets through the bi-directional coupler 18 and onto the connecting communication medium 16. Insertion loss may likewise be characterized in terms of the amount of transmitted energy from transmitter 24 that gets through the bi-directional coupler 20 onto the communication medium 16. An insertion loss of zero decibels (0 dB) would be highly desirable, however, typical insertion loss is on the order of xe2x88x9210 dB. The insertion loss of 10 dB suggests an approximate ninety percent (90%) loss of power as a normal loss through a standard bi-directional coupler.
Rejection ratio is an attribute of a bi-directional coupling device and is characterized as a measure of how much of the signal which is transmitted via the transmit port is received by (i.e., comes out of) the receive port of the same bi-directional coupler. That is, with reference to FIG. 1, rejection ratio is characterized as a measure of an amount of signal from transmitter 12 which is delivered into receiver 14 from bi-directional coupler 18 will undesirably limit the detection range of receiver 14 with respect to signals present on medium 16 which are traveling toward receiver 14.
In connection with the rejection ratio, component parts precision plays a significant role. Parts precision can be characterized as the quality of rejection that results from balancing of an electronic bridge in the particular bi-directional coupler. The electronic bridge of a bi-directional coupler is highly dependent upon a precision of the components or component parts that make it up. Parts tolerance, especially over a temperature range, is therefore a critical factor to the maintaining of a high rejection quality.
With reference now to FIG. 2, a bridge 26 from a general class of bridges used in typical bi-directional coupler devices is illustrated. That is, with typical bi-directional coupler devices, a balanced bridge 26 is employed. With the use of the bridge 26, a bi-directional coupler operates in accordance with a process for nulling, removing, or subtracting a first signal, which is to be transmitted, in such a way as to only be able to detect a second signal, which is to be received. A typical manner for nulling the transmitted signal is to use a version of the transmitted signal to cancel itself at the receive output port of the bi-directional coupler, leaving only the incoming (received) signal at the receive output port. Variations of a bridge which can be used to provide less forward signal loss but otherwise still suffers from the limitations as discussed above, are shown in FIGS. 3A, 3B, and 3C, to be discussed further herein below.
With respect to FIG. 2, the bridge 26 is made up of resistive elements 28 (Z1) and 30 (Z2) for one side 32 (or xe2x80x9clegxe2x80x9d) of the bridge 26. A resistive element 34 (Z3) and a cable impedance 36 form the other side 38 (or xe2x80x9clegxe2x80x9d) of the bridge 26. A signal to be transmitted is coupled across nodes 40 and 42 of bridge 26. A signal to be received is detected across nodes 44 and 46.
With respect to FIG. 3A, bridge 50 is made up of resistive elements 52 (R2) and 54 (R3) for one side 56 (or leg) of the bridge. The resistive element 58 (R1) and a cable impedance 60 form the other side 62 (or leg) of the bridge 50. A signal to be transmitted (denoted xe2x80x9cAxe2x80x9d) is input to bridge 50 at node 64. A signal to be received (denoted xe2x80x9cBxe2x80x9d) is output from bridge 50 at node 66. Referring still to FIG. 3A, the signal to be transmitted is further coupled to a high input impedance buffer 68, and further coupled via an isolation transformer 70 (T1).
Turning now briefly to FIG. 3B, bridge 72 is similar to bridge 50 of FIG. 3A except for the presence of additional resistive elements 74, 76, 78 and 80. Resistive elements 82 and 84 contribute to one side 86 (or leg) of bridge 72. A resistive element 88 and a cable impedance (L/O line) 90 contribute to the other side 92 (or leg) of bridge 72. A signal to be transmitted (Tx) is input to bridge 72 at node 94. A signal to be received (Rx) is output from bridge 72 at node 96. Referring still to FIG. 3B, the signal to be transmitted (Tx) is further coupled to a resistive element 98, which is further coupled to isolation transformer 100 (T1).
With reference now to FIG. 3C, bridge 102 includes resistive and impedance elements. Winding 104 of isolation transformer 106 and resistive element 108 contribute to one side 110 (or leg) of bridge 102. Resistive elements 112, 114, and 116, winding 118 of isolation transformer 106, and the cable impedance 120 (I/O line) contribute to the other side 122 (or leg) of bridge 102. A signal to be transmitted (Tx) is input to bridge 102 at node 124. A signal to be received (Rx) is output from bridge 102 at node 126.
The above described method for nulling a transmitted signal in order to detect a received signal at a bi-directional coupler 18 ultimately requires the use of a bridge. In the above-mentioned illustrations in FIGS. 2, 3A, 3B, and 3C, the bridge is made up of resistive elements R2 (Z1) and R3 (Z2) for one side (or xe2x80x9clegxe2x80x9d) of the bridge, while the resistive element R1 (Z3) and a cable impedance form the other side (or xe2x80x9clegxe2x80x9d) of the bridge. In such a bridge, there is a practical limit to the amount of signal cancellation that can be achieved with the use of standard, commercially available, component parts. Due to inaccuracy in the component parts and the resultant imperfection of the nulling process within the bridge, the signal present at the receive output port of the bi-directional coupler consists of received signal B plus a small remaining amount of transmitted signal A. Typically, with the use of standard component parts, the amount of signal cancellation is on the order of 30-40 dB of rejection. In other words, the received output of a bi-directional signal coupler would contain a signal consisting of received signal B plus a small amount of transmitted signal A. In such case, about 1-3% of the transmitted signal A will appear at the receive output port 14 of the bi-directional coupler.
The above described methods of nulling with respect to FIGS. 2 and 3 have significant limitations. For instance, the above methods suffer from insertion loss. For all practical purposes, in FIG. 2, impedance element 34 (Z3) is in series with the signal to be transmitted. Some of the energy to be transmitted is thus dissipated across impedance element 34 (Z3). As a result, a loss is created in the signal (Tx) transmitted to the cable 36.
With respect to the rejection ratio, the quality of rejection is a direct function of the balance of the particular bridge. Any variations in the balance of the bridge, as achieved in accordance with the accuracy of the component parts, will directly affect the level of rejection. If all impedance/resistive values of the bridge components are allowed a tolerance of not less than one percent (1%) of the value(s) required to achieve a perfect balance of the bridge, then transmitted signal A appearing at receive output port 14 will be reduced by not more than approximately 40 dB relative to the amplitude of transmitted signal A input to the bi-directional coupler at port 12.
Considering briefly the issue of manufacture, maintaining a given high rejection quality, especially for use over a temperature range, at a reasonable cost is quite difficult. High precision component parts which maintain their component values to high precision over a temperature range are available at high cost, thus when used in the manufacture of a bi-directional coupler, only adds to the cost of manufacture.
A primary disadvantage of the prior methods and apparatus for nulling the transmitted signal within a bi-directional coupler relates to the difficulty in achieving a substantial and reliable null of the transmitted signal at the receive output port, as well as manufacturing and component parts cost. An improved bi-directional coupler is thus desired.
The present method and apparatus solve the problems in the art by offering greater rejection with very little insertion loss, while being able to be constructed from medium tolerance component parts.
According to one embodiment of the present disclosure, a bi-directional signal coupler apparatus for the transmission and reception of signals over a single transmission medium includes a primary means for removing the transmitted signal from the received signal and at least one additional secondary means for removing the transmitted signal from the received signal. The primary means performs a first pass removal of the transmitted signal from the received signal. The primary nulling means also provides an output signal representative of the signal to be received plus the signal to be transmitted removed to a first extent from the signal to be received. The at least one additional secondary means performs a second pass removal of the signal to be transmitted from the received signal, wherein the at least one additional secondary nulling means further reduces the amount of transmitted signal present at the receive output port of the bi-directional coupler.
According to another embodiment of the present disclosure, a bi-directional transmission/reception communication system includes first and second bi-directional transmission/reception signal couplers and a single transmission medium disposed between the first and second bi-directional transmission/reception signal couplers for the transmission and reception of signals over the single transmission medium. In the bi-directional transmission/reception communication system, the first bi-directional coupler includes a primary means for performing a first pass removal of the transmitted signal, traveling through the first bi-directional signal coupler toward the single transmission medium from the signal to be received. The primary means further provides an output signal representative of the signal to be received, plus the signal to be transmitted removed to a first extent. In addition, at least one additional secondary means is provided for performing a second pass removal of the transmitted signal from the signal to be received, wherein the at least one additional secondary means operates upon the output signal of the primary means and is selected to achieve a desired further reduction in the amplitude of the transmitted signal appearing at the receive output port of the first bi-directional coupler.
Still further, according to yet another embodiment of the present disclosure, a method for bi-directional signal coupling the transmission and reception of signals over a single transmission medium is disclosed. The bi-directional coupling method includes the steps of providing a primary means for removing a signal to be transmitted, traveling through a bi-directional signal coupler, from a signal to be received. The primary means further provides an output signal which is representative of the signal to be received, plus the signal to be transmitted removed to a first extent. The method further includes the step of providing a secondary means for removing the signal to be transmitted from the signal to be received. The secondary means operates upon the output signal produced by the primary means and is selected to achieve a desired further reduction in the amplitude of the transmitted signal relative to the signal to be received. The method still further includes the step of providing at least one additional secondary means for removing the signal to be transmitted from the signal to be received. The at least one additional secondary means operates upon an output signal produced by the first secondary means and is selected to achieve a desired further reduction in the amplitude of the transmitted signal appearing at the receive output port of the bi-directional coupler.